Innovating microstructured gas-liquid-solid reactors : a contribution to the understanding of hydrodynamics and mass transfers

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contribution to the understanding of hydrodynamics and

mass transfers

Jean-Noël Tourvieille

To cite this version:

Jean-Noël Tourvieille. Innovating microstructured gas-liquid-solid reactors : a contribution to the

understanding of hydrodynamics and mass transfers. Chemical and Process Engineering. Université

Claude Bernard - Lyon I, 2014. English. �NNT : 2014LYO10030�. �tel-01015051�

N° d’ordre: 30-2014 Année 2014

Thèse

présentée

devant l’UNIVERSITE CLAUDE BERNARD-LYON 1

pour l’obtention du

DIPLÔME DE DOCTORAT

(arrêté du 07 août 2006)

Ecole doctorale de Chimie

Spécialité: Génie des procédés

Présentée et soutenue publiquement le 26/02/2014

par

Jean-Noël TOURVIEILLE

Ingénieur ESCPE Lyon

Innovating microstructured gas-liquid-solid reactors: A

contribution to the understanding of hydrodynamics and mass

transfers

Directeur de thèse: M. Claude de Bellefon

Jury:

Madame Joëlle AUBIN

Monsieur Michel KREUTZER

Monsieur Pascal FONGARLAND

Monsieur Patrick MAESTRO

Madame Lioubov KIWI-MINSKER

Monsieur Régis PHILIPPE

Président de l’Université

Vice-président du Conseil d’Administration

Vice-président du Conseil des Etudes et de la Vie Universitaire Vice-président du Conseil Scientifique

Directeur Général des Services

M. François-Noël GILLY

M. le Professeur Hamda BEN HADID M. le Professeur Philippe LALLE M. le Professeur Germain GILLET M. Alain HELLEU

COMPOSANTES SANTE

Faculté de Médecine Lyon Est – Claude Bernard

Faculté de Médecine et de Maïeutique Lyon Sud – Charles Mérieux

Faculté d’Odontologie

Institut des Sciences Pharmaceutiques et Biologiques Institut des Sciences et Techniques de la Réadaptation

Département de formation et Centre de Recherche en Biologie Humaine

Directeur : M. le Professeur J. ETIENNE Directeur : Mme la Professeure C. BURILLON

Directeur : M. le Professeur D. BOURGEOIS Directeur : Mme la Professeure C. VINCIGUERRA Directeur : M. le Professeur Y. MATILLON Directeur : M. le Professeur P. FARGE

COMPOSANTES ET DEPARTEMENTS DE SCIENCES ET TECHNOLOGIE

Faculté des Sciences et Technologies Département Biologie

Département Chimie Biochimie Département GEP

Département Informatique Département Mathématiques Département Mécanique Département Physique

Département Sciences de la Terre

UFR Sciences et Techniques des Activités Physiques et Sportives Observatoire des Sciences de l’Univers de Lyon

Polytech Lyon

Ecole Supérieure de Chimie Physique Electronique Institut Universitaire de Technologie de Lyon 1 Institut Universitaire de Formation des Maîtres Institut de Science Financière et d'Assurances

Directeur : M. le Professeur F. DE MARCHI Directeur : M. le Professeur F. FLEURY Directeur : Mme le Professeur H. PARROT Directeur : M. N. SIAUVE

Directeur : M. le Professeur S. AKKOUCHE Directeur : M. le Professeur A. GOLDMAN Directeur : M. le Professeur H. BEN HADID Directeur : Mme S. FLECK

Directeur : Mme la Professeure I. DANIEL Directeur : M. C. COLLIGNON Directeur : M. B. GUIDERDONI Directeur : M. P. FOURNIER Directeur : M. G. PIGNAULT Directeur : M. C. VITON Directeur : M. A. MOUGNIOTTE

Lycéen dans une section technologique, je ne me destinais pas a priori à connaître le monde de la recherche académique. Réaliser un doctorat aura donc été le point d’orgue d’un parcours parfois difficile, mais très enrichissant.

Enrichissant avant tout par les rencontres qu’il m’a été données de faire. Au LGPC, elles ont été nombreuses et m’ont permis d’évoluer et d’apprendre autant en science, qu’en technique. Mais avant tout, cettet thèse est aussi le fruit de collaboration, où les relations humaines ont toutes leur place, et font apprendre plus parfois que le reste.

Merci à toi Frédéric, pour ta patience, ta pédagogie et ta bonne humeur quotidienne. Dans les moments difficiles, ça a toujours été un réconfort. Je pense que ça le sera aussi pour d’autres étudiants encore1 Merci à vuos Marie-Line et Stéphanie, sans qui la vie dans le labo ne serait pas ce qu’elle est : dynamique et joviale. Sans compter que partager ce bureau avec toi Marie-Line a été la meilleure chose dans la thèse ! Stéphanie, je n’oublirai pas ce voyage au Maroc ensemble. Que de bons moments!

Merci à toi aussi Fabrice, sans qui nos idées ne seraient restées que sur papier. Merci Laurent, pour tes remarques inoubliables, tes méthodes GC géniales et tes conseils boursiers !

Merci à tous les autres qui ont pu m’aider directement ou indirectement. Valérie, qu’aurais-je fait sans tes méthodes de dépôts de catalyseur ? Isabelle, que dire de nos longues conversations ? Et de ce voyage à Pékin ? Je ne pourrai pas non plus oublier l’aide précieuse de Régis, pour m’aider à rentrer dans ce monde qu’est la recherche. Nous avons parfois cherché tous deux des réponses à nos observations. Ta rigueur scientifique et ton ouverture humaine m’ont beaucoup appris.

Et bien sûr Claude, merci pour m’avoir donné ma chance. Je sais que vous avez du faire preuve de patience avec les erreurs que j’ai pu faire. Sans nul doute, votre disponibilité, votre esprit constructif et votre confiance ont été des artisans de ces travaux.

A ma famille, merci de votre compréhension pour toutes ces heures passées à travailler plutôt qu’à être avec vous. Ce travail vous est dédié.

A toi aussi Charly, pour avoir su, ou du moins essayé, de me faire prendre du recul et m’avoir supporté. Même si tu étais à Copenhague, nous avons embarqué dans la même galère tous les deux !

Enfin à toi, Myriam, qui est devenue ma femme durant ce doctorat. Un grand merci pour ta patience dans les moments difficiles (et il y en a eu !), ta générosité et ton moral si fort, même dans l’adversité. Je te dois beaucoup.

To meet the new challenges of the chemical indutries, the developpement of new heterogeneous catalytic reactors and their understanding are mandatory. From these perspectives, new reactor designs based on structuring at micro or millimeter scales have emerged. They have sparked interest for their ability to decrease physical limitations for heat and mass transfers. Thus, two advanced reactor technologies for gas-liquid-solid catalysed reactions are studied. The first reactor is a micro-structured falling film (FFMR) in which vertical sub millimetric grooves are etched and coated with a catalyst. This structuration allows stabilizing the gas-liquid interface of a down flow liquid phase. A thin liquid film is generated leading to high specific surface areas. Commercially available, it represents a very good potential for performing demanding reactions (i.e.fast, exothermic) for small scale productions such as pharmaceuticals. In a second part, a new reactor concept is proposed. Open cell foams are used as catalyst support and inserted in a milli-square channel. The reactor is then submitted to a preformed gas-liquid Taylor flow. In both cases, hydrodynamics features are studied by using microscopy based methods. Their potential in terms of mass transfers are also studied by performing catalyzed Į-methylstyren hydrogenation. For both reactors, it comes out that the particular flow induced by micro or milli structures leads to at least one order of magnitude higher mass transfers performances than mutliphase reactors currently used in the industry albeit it remains to be demonstrated at such scale. From all these studies, correlations, models and methods for chemical engineers (hydrodynamics, pressure drops, mass transfer) are proposed for the two reactors.

Key words: Microstructured falling film ; Open cell foams ; Gas-liquid-solid; Milli channel ; Microscopy ; Liquid film profile ; Hydrodynamics ; Mass transfers ; Pressure drop

Afin de répondre aux nouveaux challenges de l’industrie chimique, le développement de nouveaux réacteurs catalytiques hétérogènes plus efficaces et plus surs ainsi que leur compréhension sont nécessaires. Dans cette optique, des réacteurs micro ou milli-structurés ont vu le jour et suscitent un intérêt croissant de par leur capacité à diminuer les phénomènes physiques de limitations aux transferts de mantière et de chaleur. Dans ce travail, deux concepts de réacteurs structurés dédiés au milieu gaz-liquide solide sont étudiés. Le premier est un réacteur à film tombant microstructuré (FFMR) dans lequel des canaux sub-millimétriques, rectilignes et verticaux permettent de stabiliser et d’amincir un film liquide en écoulement, générant des aires d’interfaces très importantes. Disponible commercialement, il présente un très bon potentiel pour la mise en oeuvre de réactions à fortes contraintes mais pour de petites productions. Le second réacteur est quant à lui nouveau. Des mousses à cellules ouvertes métalliques sont utilisées comme support de catalyseur structurant confiné dans un canal de section millimétrique carrée et soumis à un écoulement de Taylor G-L préformé. Pour chaque réacteur, l’hydrodynamique des écoulements est étudiée par le développement de techniques microscopiques et leurs aptitudes aux transferts de masses sont évaluées par la mise en oeuvre de la réaction catalytique d’hydrogénation de l’Į-methylstyrène. Il en ressort que les écoulements particuliers rencontrés dans ces deux objets permettent d’atteindre des capacités de transferts de matières supérieurs d’au moins un ordre de grandeur aux technologies usuelles pour un coût énergétique, lié à l’écoulements des fluides, faible. Par ailleurs, des éléments de dimensionnement (hydrodynamique, perte de charge et transferts de matière) ont été construits pour les deux réacteurs.

Mot clés : Film tombant micro structuré ; Mousses métalliques, Gaz-liquide-solide ; Milli canal ; Profile de film liquide ; Microscopie ; Transferts de masses ; Pertes de charges

DISCIPLINE : Génie des procédés catalytiques

INTITULE ET ADRESSE DE L'U.F.R. OU DU LABORATOIRE :

Laboratoire de Génie des Procédés Catalytiques - CNRS/CPE Lyon 43, bd du 11 novembre 1918, B.P. 82077, 69616 VILLEURBANNE Cedexȱ

Chapter 1: Introduction ... 1

Preamble ... 1

1.1. Introduction to multiphase reactions ... 1

1.2. Process intensification... 3

1.3. Concepts of interests ... 12

1.4. Scope and outline of the thesis ... 15

1.5. References: ... 17

Chapter 2 : Mass transfer characterization of a microstructured falling film at pilot scale ... 20

2.1. Introduction ... 20

2.2. Methods, experimental set-up and procedures ... 23

2.2.1. Visualisation of the liquid film ... 23

2.2.2. Overall G-L-S mass transfer ... 24

2.3. Results and discussions ... 27

2.3.1. Liquid film study ... 27

2.3.2. Overall mass transfer coefficient ... 32

2.4. Conclusions and perspectives... 35

2.5. Nomenclature ... 36

2.6. References ... 37

2.7. Appendix ... 38

Chapter 3 : Effect of channel dimensions on the performances of a micro-structured falling film at pilot scale ... 40

3.1. Introduction ... 40

3.2. Methods and experimental aspects ... 43

3.2.1. Confocal microscopy... 43

3.2.2. Mass transfer experiments ... 46

3.3. Results and discussions ... 48

3.3.1. Liquid film study ... 48

3.3.2. Estimation of mean residence time and G-L specific surface area ... 55

3.3.3. Estimation of mass transfers ... 57

3.4. Conclusions and perspectives... 60

3.5. Nomenclature ... 61

3.6. References ... 62

3.7. Appendices ... 63

Chapter 4 : Hydrodynamics in a milli square channel filled with metal foams submitted to a periodic flow: Flow patterns, residence time distributions and pulsing properties ... 69

4.1. Introduction ... 69

4.1.1. Flow regimes and liquid hold-up ... 70

4.1.2. Liquid dispersion and back mixing... 71

4.1.3. Aim of this work and strategy ... 72

4.2. Methodology and theory ... 72

4.2.1. Foam morphology characteristics... 72

4.3.1. Reactor design ... 76

4.3.2. Foam properties ... 76

4.3.3. Flow patterns and pulsing regime studies... 77

4.3.4. RTD measurement ... 78

4.4. Results and discussions ... 80

4.4.1. Flow patterns ... 80

4.4.2. Flow patterns with fluorescent μ-particles... 83

4.4.3. Pulsing regime characteristics ... 85

4.4.4. RTD ... 93

4.4.5. Liquid hold up ... 100

4.5. Conclusions and perspectives... 103

4.5.1. Flow patterns ... 103

4.5.2. Residence time distribution and RTD... 104

4.6. Nomenclature ... 105

4.7. References ... 106

4.8. Appendices ... 108

Chapter 5 : Mass transfer performances of a milli-reactor containing catalytic foams in pulsing regime ... 127

5.1. Introduction ... 127

5.1.1. Background on heterogeneous catalytic micro- or milli- structured reactor... 127

5.1.2. Open cell foam as catalyst support ... 128

5.1.3. Scope of this work : ... 129

5.2. Methodology and theory ... 129

5.2.1 Foam morphology characteristics... 129

5.2.2 Pressure drop ... 129

5.2.3 Overall mass transfer estimations ... 132

5.3. Experimental ... 134

5.3.1 Reactor characteristics ... 134

5.3.2 Foam characteristics ... 135

5.3.3 Pressure drop measurement ... 135

5.3.4 Catalyst deposition ... 135

5.3.5 Mass transfers experiments... 137

5.4. Results ... 138

5.4.1 Pressure drop: ... 138

5.4.2 Global mass transfer coefficient ... 142

5.5. Comparison with other technologies ... 149

5.6. Conclusions and perspectives... 152

5.6.1 Pressure drop ... 152

5.6.2 Reactor performances ... 152

5.7. Nomenclature ... 152

5.8. References ... 154

5.9. Appendix: ... 156

Chapter 6 : Conclusions and Outlook ... 159

6.1 Conclusions ... 159

6.2 Recommendations ... 160

Introduction

Preamble

This thesis is part of the European project Polycat which aims to develop new polymer based catalysts with nan particles for the pharmaceutical industry, crop protection and vitamin syntheses in conjunction with intensified process technologies and green solvents. This will lead to the replacement of a number of chemical steps of fine chemical syntheses by catalytic steps using more active, selective and stable catalysts. To achieve these goals, it has to provide a discipline bridging approach between fine chemistry, catalysis and engineering. Therefore, this project involves 18 partners from different backgrounds and fields.

Amongst them, 7 research centres are involved in the development of catalysts and their characterizations (Helmotz-Zentrum-Berlin, University upon Tyne, Foundation of research and Technology Hellas, école polytechnique fédérale de Lausanne, Abo akademi, Tver Technical University and the institute of Organoelement Compounds of Russian Academy Science). 3 companies are involved in their integration in advanced reactors (Institut für Mikrotechnology Mainz (IMM), Thalesnano, and Picosun) and 2 research centres focuse on the chemical engineering understanding of these advanced reactors (Laboratoire de Génie de Procédés Catalytiques (LGPC), and école polytechnique fédérale de Lausanne). Furthermore, the participation of industrial companies is needed to get the opportunity to work on industrial cases and to operate a transfer from academic concepts world to industrial production. To provide this market push, 2 companies (Sanofi (Vitry) and Bayer Technology Service) have joined.

One of the main tasks entrusted to LGPC is the study of micro structured reactors for multiphase reactions. Some reactors have been developed by other project partners such as IMM, and a more fundamental approach is sought to develop tools for their use. In the mean time, the design and the study of a new reactor are undertaken.

1.1. Introduction to multiphase reactions

General considerations

The production of consumer end-products and intermediates are the result of phenomena occurring in the reactor. This is the heart of the process, where reactants are transformed. Reactants are often present in different phases such as gas and liquid, which lead to multiphase systems. In many cases, a metal deposited on an inert support is also required to accelerate the reaction and increase the selectivity of the desired product. These so called gas-liquid-solid systems are widespread in the industry and can be found in diverse areas such as manufacture of petroleum based products, fuels, pharmaceuticals, intermediates chemicals, polymers, remediation and so on. The overall efficiency of the process is the result of the combination of numerous physical and chemical phenomena. To satisfy to the production outcomes, coupling both kinds of phenomena to design the most suitable reactor is needed. Therefore, prior to a good description of the reactor, a good

understanding of theses processes is needed. This approach has led to various reactor technologies as reviewed by Dudukovic et al. (2002). Typical reactions are:

- Fischer Tropsch synthesis, in the field of energy - Hydrodesulfurization of petroleum feed stock - Hydrogenations (eg. of nitro groups)

- Oxidations (eg. cyclohexane to adipic acid)

These reactions are known for their particular requirements, such as strong exothermicity, fast kinetics or for their explosivity.

Gas-liquid-solid reactions

These reactions can be described as the result of several steps in series or in parallel, each step having potentially large influence on the global process.

The steps are described as follows (Figure 1.1):

- Transport of the gas reagent in the gas phase - Dissolution of the gas reagent into the liquid phase

- Transport and diffusion in the liquid phase to reach the catalyst surface - Once at the surface, diffusion into the catalyst pore network to reach active site - Absorption on the active site and reaction

- Desorption from the active site - Diffusion out of the pore network

The apparent rate of the reaction will be determined by the rate of the slowest step. For slow intrinsic kinetics, the reaction is only dependant of the kinetic law and the amount of catalyst. Increasing the amount of catalyst leads to increase the reaction rate until the characteristic reaction time becomes lower than the mass transport time. Under such conditions, the reaction rate starts to be solely dependant on resistances encountered by reactants to reach the active site. These resistances can be represented as resistances in series (Figure 1.1).

Gas Liquid Solid

klsals k_gla_gl k_ga_gl Internal diffusion & reaction

Gas Liquid Solid

Gas Liquid Solid

klsals k_gla_gl k_ga_gl Internal diffusion & reaction

Figure 1.1: Illustration of the concentration profile of reactant in different phases

The reaction rate will increase if the mass transfer coefficients k and the specific surface areas a are large. Mass transfers rates can be increased by choosing a suitable way to put phases in close contact. The catalyst support

Chemical processes

can be dispersed in the liquid phase or fixed in the reactor, depending on the requirements of the process (eg. exothermicity, deactivation). In industry, many forms of catalysts have been designed to provide high specific surface areas, as well as to promote flow convection through structuring in fixed bed operations

(

).

Figure 1.2 : Illustration of various kinds of catalyst support design (a) developed by Süd Chemie (b) developed by Axens (c)

developed by Zibo Xinhengli

Choosing a suitable catalyst shape has to be done taking into account associated constraints, such as mechanical strength, pore volume and pressure drop generated. For example, decreasing diameter of spherical particles will provide higher exchange surface area but also much higher pressure drop.

Thus, to achieve better process performances, the development of new catalyst supports is needed. Nevertheless, this is not the only way to explore. Increasing mass transfers can also be achieved by considering the reactor design itself. All these aspects lead to the process intensification concept.

1.2. Process intensification

The aspects discussed previously give strong motivation to the chemical engineering community for a paradigm shift and lead to what is mentioned now as process intensification. As noted by van Gerven and Stankiewicz (2009), the term ”process intensification“ is nowadays commonly evoked although it suffers from unclear and various definitions. They identified four principles on which process intensification is based:

- To maximize the efficiency of intra and extra molecular events

This includes the use of catalyst as described previously but also any means to increase selectivity.

- To give the same processing experience for each molecule

This mainly concerns residence time distribution, mixing properties and heating homogeneity. As an example, a broad residence time distribution of molecules may lead to severe drawbacks on selectivity depending on the reaction scheme. Similar consequences come from hotspots present in catalyst bed.

- To increase driving forces at each scale level

As described previously, reaction rates may be governed by mass transport phenomena. Decreasing the characteristic length of reactor or catalyst support leads to shorter characteristic time for the mass transfer. It also provides a high surface to volume ratio.

- To maximize the synergistic effects from partial processes

Reactive distillation is a good example to quote: the reaction equilibrium is shifted by taking away in situ products generated.

Based on the third principle, the advent of milli or micro reactors has greatly contributed to the paradigm shift evoked above. Due to their small dimensions, high surface to volume ratios are provided. Thus, high heat and mass transfers are achieved. Safety is also greatly improved. Indeed, material inventory are very low and radical chain propagation is limited by the proximity of reactor walls (Veser, 2001).

Micro-structuring – scaling down

At these scales, the predominant forces are not the same as in macro scale reactors. To understand the influence of forces on the system it is necessary to have a look to characteristic time related to them. Table 1.1 gives the expressions for the characteristic times.

Gravity Inertia Capillary Viscosity

g L u L σ σ σ σ ρ ρρ ρ 3 L µ µµ µ ρ ρρ ρ_L3

Table 1.1: Characteristic time relative to the forces

(L is the characteristic length, g is the gravity, u is the liquid velocity, ı is the surface tension and

ȝ is the viscosity)

The lower the time is, the higher is the rate of establishment of the phenomenon and then, its relative importance compared with others. At the meter scale, gravity and inertial forces are predominant on viscous force and surface tension. Figure 1.3 illustrates qualitatively this remark.

Gr av it ati o nn al In er tial S u rf ac e Ten s io n Viscou s 1 m 100 μm Grav it a ti o nal In er tial S u rf ac e Ten s io n Viscou s Gr av it ati o nn al In er tial S u rf ac e Ten s io n Viscou s 1 m 100 μm Grav it a ti o nal In er tial S u rf ac e Ten s io n Viscou s

Figure 1.3 : Influence of scale on the role of forces (from Leclerc, 2007)

By scaling down the reactor, a shift appears. Viscous forces and surface tension become predominant because of the dependence on L3/2 and L3 of the capillary and viscous characteristic time expressions. This has a great impact on hydrodynamics and flow patterns encountered in these reactors. Most of the rules used in the design and understanding of conventional scale reactors are not applicable here and new ones have to be established. Over the past decade, many concepts based on microstructuring have been developed for gas-liquid systems and tested for various syntheses (Kashid et al., 2009). When turning to heterogeneous systems, solid management makes the design and the use of micro structured devices more challenging.

Basically, there are two approaches for bringing two phases into contact. One phase can be dispersed in another by using a suitable inlet device or to keep phases into continuous contact. The first configuration involves a phase separation stage downstream. In small channels, several flow regimes are observed with very different properties (Figure 1.4). At low gas flow rates and in conditions where coalescence is low (moderate liquid flow rate), bubbles are dragged away by the liquid phase. This flow pattern is commonly called bubbly flow. For reaction purposes, it is not really interesting as the gas availability is very small and may limit the reaction. When increasing the gas flow rate, Taylor flow or slug flow regime is obtained (Figure 1.4.c and Figure 1.4.d). Under theses conditions, the gas forms a train of bubbles separated by liquid slugs. Gas bubbles occupy almost all the channel section. Depending on the wetting properties, only a thin liquid film stays between the gas bubble and the channel walls. This flow regime benefits from increasing interest for its hydrodynamics properties. Indeed, exchange of fluids between two successive slugs is only done through the liquid film, reducing considerably the back mixing.

Figure 1.4 : Observed flow regime in capillary tubes (a, b) bubbly flow (c, d) segmented flow (e) transitional slug/churn

flow (f) churn flow (g) film flow (in down flow configuration only) (h) annular flow (from Kreutzer et al.2005)

Furthermore, recirculations appear in the liquid slug, favouring mass transfers. The length of bubbles and slugs is determined by inlet conditions and the injector shape. Most of applications in micro channel use this flow regime. At higher gas velocities, a chaotic flow appears which is called churn flow. The gas-liquid interface is not well defined anymore and the system tends to turn into separate phases due to the strong interaction between phases. At lower liquid hold up, an annular flow is obtained. This flow is characterized by the presence of a wavy liquid film which is dragged by a gas core. Solvent striping has also to be considered. Despite a broadening of the liquid residence time distribution and the high amount of liquid dragged into the gas phase, this regime is interesting for the very high specific surface and the large amount of gas available. If the drawbacks are not limiting steps, this regime can be used for reaction purposes as demonstrated by Chambers et al. (2001).

The other way to bring the phases into contact is to keep the phases into continuous contact. In this configuration, fluids are fed and collected separately. The key step is the control of the interface.

In the following, some recent technologies based on these two contact modes (dispersed and continuous) are presented and discussed. The purpose is not to give an exhaustive review of gas-liquid-solid micro structured reactors but rather an overview of recent progresses in multiphase micro contactors and their main advantages and drawbacks.

Gas-liquid-solid reactions in micro-structured reactors Continuous phases:

Falling Film Micro Reactor (FFMR)

Initially developed by IMM for gas-liquid reactions such as direct fluorination of toluene (Janisch et al., 2000), sulfonation of toluene (Müller et al., 2005) amongst others, the reactor is composed of a vertical plate on which parallel micro channels are etched. The liquid flows downward due to gravity while gas flows at co or counter current. Structuring allows stabilizing the gas-liquid interface and a very thin liquid film is generated, with thicknesses of the order of 15 to 150 μm compared to 3 mm for industrial falling film reactors. Thus, very high gas-liquid surface are generated (up to 20 000 m2.m-3 mliq) and high mass transfers are achieved. The back side of the plate is used as a heat exchanger by flowing cooling or heating utilities.

Figure 1.5 : Falling film micro reactors (a) lab scale and pilot scale reactor (b) structured plate for the two scale reactors

Here the thickness of the liquid film is controlled by the channel dimensions and the liquid flow rate. Various channel dimensions are available: 100 x 300 μm2_{; 200 x 600 μm}2_{; 400 x 1200 μm}2

(Depth x Width). It is possible to coat the channels surface with a catalyst as demonstrated by Yeong et al. (2003, 2004) who performed nitrobenzene hydrogenation. They estimated mass transfers roughly ranging from 0.5 to 8 s-1 which outperform most of reactors known. Despite that catalyst deactivation was observed, it proves that with suitable coating methods it is possible to use this system for gas-liquid-solid reactions. Production in the order of kg per day is possible for rather fast reactions (eg. nitrobenzene hydrogenation).

The main drawback of the FFMR is scaling up. In the same way as other multi channel tools, maldistribution issues are made more important and can even be critical. Thus, thorough attention has to be paid to the design of the liquid injector. Another bottleneck is the short residence time in the reactor, in the range of 10-30 s. It can be tuned either by increasing reactor height or by tilting it. However, these configurations can be drastically unfriendly to the end users.

Finally, different reactor scales are available but only the lab scale version (Figure 1.5) has been subject to more thorough understanding.

Mesh Reactor (MR)

Figure 1.6 : Mesh reactor (a) Fully assembled apparatus (b) micro structured mesh (adapted from Abdallah et al., 2006)

In this reactor, a structured mesh is used to separate fluid flows and to stabilize the gas-liquid interface. The mesh to wall distance can be set to 80-140 μm giving a chamber volume of about one hundred microliters. The pore width is in the range 1-10μm and the ratio of pore length to width is ~ 1. This configuration makes it much easier to control the liquid phase than in the FFMR case. Then, the residence time can range between few seconds to several minutes. To minimize mixing between two samples, a radial flow configuration from an entrance port that flushes the whole reactor is used.

In the same manner as for the FFMR, it is possible to coat the liquid side plate with a catalyst. Abdallah et al. (2006) performed hydrogenation of Į- methylstyrene and CFD to investigate reactor performances. They found global G-L-S mass transfer coefficients about 1 - 2 s-1 , which is much higher than for industrial reactors.

Nevertheless, the main limitation comes from the stability of the interface at high pressure. When the working pressure overcomes the capillary pressure, the gas phase can go through the mesh and destabilize the interface.

Dispersed phases

Micro packed bed reactor (μ-PBR)

Losey et al. (2001) developed a micro packed bed reactor and used the hydrogenation of cyclohexene over platinum as a model reaction. Ten parallel channels are made from silicon. The catalyst is delivered as slurry by two ports allowing cross flow. Gas and liquid are split into those channels where different flow regimes - segmented or dispersed - are observed.

Figure 1.7 : Multi channel reactor chip (adapted from Losey et al.,2001)

The global G-L-S mass transfer coefficient estimated with the model reaction is in the range 5-15 s-1. But the price to pay is the very high pressure drops generated (~ 1-10 atm/m) even for low liquid flow rates.

On the other hand, the very high surface to volume ratio is clearly favourable for heat control in the reactor.

Pillar reactor (PR)

In the continuity of packed bed, Jensen (2001) mentioned the use of micromachining techniques to design structured pillars bed. It offers an alternative to random packed bed with lower pressure drop and still the possibility to turn it catalytically active by wash coating and chemical surface treatment. This configuration provides very high specific surface area in the range 4-10. 104 m2.m-3channel which obviously is favourable to mass transfers (De Loos et al. 2012). Nevertheless, machining and numbering up such reactors can be very costly and not suitable for production purposes even at lab scale.

Figure 1.8 : Example of structured packing bed made by DRIE (from Jensen, 2001)

String reactor (SR)

It follows the principle of packed bed reactors but here the catalyst particles have almost the same dimension as the channel. This allows a reduced pressure drop compared with smaller particles and still provides rather high solid hold-up and solid specific surface areas. Besides, same gas-liquid flow regimes as found in open microchannel are possible (bubbly, slug, annular).

Figure 1.9 : Illustration of string reactor concept; two ways to pile up sphere particles (from Haase et al., 2013)

Hipolito et al. (2010) characterized this reactor in a horizontal configuration with channel diameter of 2-4 mm. At these dimensions gravity starts to play a significant role as high slip velocity between phases are obtained. Thus, they found global mass transfers Kov in the range of 0.1-0.5 s-1 using the Į –methylstyrene hydrogenation. Following the same concept, Haase et al. (2013) used a vertical configuration and smaller channel dimension (1mm). With the same reaction but at higher temperature, they obtained Kov values up to 4 s-1.

Slurry Taylor

For most of reactors presented previously, deactivation is a severe bottleneck as it is difficult to regenerate the catalyst in-situ, or to change the catalyst support (eg. FFMR structured plate) or even the entire reactor (Monolith). To cope with this issue it has been suggested to introduce solid particles in Taylor slug flow (Figure 1.10). This concept has been validated by Liedtke et al. (2013) who performed hydrogenation of 3 methyl-1-pentyn-3-ol as a model reaction. They showed that they were under kinetic control while this reaction exhibits a rather high reaction rate. In addition to the interesting properties of Taylor flow (low axial dispersion, good radial mixing, and high G-L mass transfers) it offers an elegant way to handle rather easily the catalyst circulations.

Obviously, here the main parameters are the particles properties and the flow conditions to keep the particles in motion. Recirculation in the liquid slugs has to be strong enough to ensure a good dispersion of the solid. Finally, even if gas and liquid phases behave as plug flow, this is not necessarily the case for the solid phase. Back mixing occurs through the liquid film where particles are immobilized while the bubble is passing. Thus, solid RTD may be impacted and can have critical impact on reactor performances in case of deactivation. Another limitation may come from the small size of particles which could plug upstream or down stream operations.

Figure 1.10 : Image capture of the slurry Taylor flow (adapted from Liedtke et al., 2013)

Structured packing

Various kinds of structured supports have arisen over the years. The most used are monolith structures which are hundreds of straight channels made from the same piece of material (e.g. by extrusion). This type of contactor usually operates under Taylor flow although annular flow regime is also possible. Monolith reactor found applications in many fields such as hydrogen peroxide synthesis via alkylanthraquinone (Edvinsson et al., 2001), Fischer-Tropsch synthesis (Hilmen et al., 2001), and methanol synthesis (Cybulski et al., 1993) amongst others. External surface areas ranging from 1500 to 2500 m2.m-3 are provided leading to high gas-liquid mass transfers with kLa value up to 1s-1. On the other hand, liquid solid mass transfers are much lower with klsals in the range 0.03 - 0.09 s-1 (Cybulski and Moulijn, 2006). One major bottleneck is that scale up may be an issue due to the difficulty to ensure homogeneous distribution of gas and liquid flows. This problem is inherent to any multi channel technology.

Other structured packings can also be included in the field of micro-structured reactors which have not been discussed yet such as Mellapak packing and open cell metal foams in particular (Figure 1.11). These latter have been subject of some research for gas-liquid-solid reactions in macro scale reactors (Stemmet et al., 2008; Wenmakers, 2009; Tschentcher et al., 2010).

Nevertheless, these objects are used in reactors with inner diameter over few centimetres. Thus, the structuring effect of walls - encountered in reactor previously described - is not significant. To stay in the logical sequence of the presentation, these objects are not considered further for the moment.

Reactor characteristics summary

This section addresses a summary of performances characteristics for presented reactors according to data found in the literature (Table 1.2). For a fair comparison, mass transfers are corrected to 298K. Many other technologies could have been included but a particular attention is given to these reactors as they all promote structuring of the flow patterns due to the geometry and dimensions of the reactor/channels. Thus, foams and packed beds with internal diameter over few centimetres are discarded.

1 1

www.sulzer.com

Figure 1.11 : Different structured packings (a) Mellapak1 packing (b) 3D tomography reconstruction of open cell foam packing (internal work)

From this summary, two promising technologies appears. Continuous phase reactor FFMR seems to be interesting for its high mass transfer abilities (Kov ~ 2-6 s-1) and the very low pressure drop generated. This kind of contactor are to be privileged when high gas flow rates are needed or for rather small productions, as example in the pharmaceutical industry.

On the other hand, wall coated monolith are very attractive for larger productions. They present good overall mass transfer performances for low pressure drop. An evolution of this reactor consists in filling channels with beads with large diameter compared to channel dimensions as in the string reactor, even if this concept has never been undertaken at pilot scale. It gives better mass transfer performances than hollow channels but the price to pay is a higher pressure drop. Also, in both cases the catalyst management may be difficult in particular in the case of a strong deactivation. For this latter case, a multi-tubular slurry Taylor would suit the best. Finally, despite very interesting mass transfer performances, the μ-PBR exhibits too high pressure drops for too low production.

Concluding remarks

Despite all interests generated by the reactors described above, most of them are still at the academic stage or suffer from shortcoming knowledge for a breakthrough in the industry. Nevertheless, two concepts are catching our attention: that used in the FFMR developed by IMM and that involving the use of millimeter scale internal structures as catalyst supports (monolith, string reactor).

2

agl or als based on liquid volume 3 FFMR a Mesh reactorb μ-packed bed c Slurry Taylor reactor d String reactor e Monolith f

Contact mode Cont.2 Cont.1 Disp.3 Disp.2 Disp.2 Disp2

agl (m-3reac or liq) up to 3.104 2500 -. - - - als (m-3reac or liq) 1-3.104 6500 1.5-3.104 600-5400 500-1500 1500-2500 Kov (s -1 ) (298K) 2-6 1-2 2-6 0.04-2 0.25-3 0.03-0.12 ǻP/L (KPa.m-1 ) N.S. N.S. 500-3000 <10 40-60 <10 İL (m3.m-3reac) <0.02 <0.02 0.15-0.27 0.25-0.75 0.25-0.75 0.25-0.75 Catalyst load + - ++ + + + Catalyst management (case of deactivation) -- -- + ++ + -- Fluids management + ++ - -- - -- Potential production + - -- + + ++ Scaling out + + -- - - +

Table 1.2 : Micro-structured reactor characteristics for gas-liquid-solid reactions. Values estimated from (a) Internal work

(b) Abdallah et al. (2006) (c) Losey et al.(2001); Faridkhou et al. (2013) (d) Internal results not published yet (e) Hasse et al. (2013) (f) Kreutzer et al. (2001); Cybulski and Moulijn (2006)

Commercial versions are already available for the FFMR but fundamental understandings are still not complete enough to propose design rules for specific case. Besides this, all studies found in the literature use a lab scale version of the reactor. At pilot scale, everything remains to be studied.

The second concept is based on the idea of improving monolith channel reactors by filling channels with a suitable packing. As evoked above for the string reactor, the use of beads enhances mass transfer performances but for higher pressure drop. An alternative for minimizing pressure drop would be the use of structured packing having high porosity (İ ~ 0.8-0.9).

These two concepts will be further discussed in what follows.

1.3. Concepts of interests

Falling film micro reactor at pilot scale

As it was stated previously, the control of the gas-liquid interface and the liquid film thickness are key parameters in bicontinuous phase reactors. The understanding of the location of the three phase contact line and how it behaves with liquid properties and channel geometry/dimensions is a key to set design rules. In the original version of the IMM reactor, the manufacturing process is electro discharge machining or wet etching. This leads to semi-circular or semi-elliptic channel profiles. The typical profile is illustrated on a channel slice with a catalyst coating (Figure 1.12).

Figure 1.12 : MEB picture of a channel slice (a) plate (b) Ȗ- Al2O3 coating (Stavarek e al., 2008)

The depth to width ratio was fixed at approtximately three for several channel dimensions. One pending question is the shape of the gas-liquid interface in the channel. According to the liquid properties and liquid flow rate, several possibilities are foreseen albeit not being exhaustive (Figure 1.13).

?

?

Figure 1.13 : Hypothetic liquid film profile with different liquid properties and flow rate (a) multiple location of the three

phase contact line (b) three phase contact line pinned at the top of the channel wall (c) large channel: absence of wall effect (d) liquid with high contact angle (e) situation just before flooding (f) others…

Some answers have already been given by Yeong et al. (2004) who used confocal microscopy in reflexion mode to get the liquid profile for various liquid conditions and properties as well as channel dimensions. This method gave good results when the curvature of the liquid menisci is low enough to discriminate the liquid from the wall, thus for rather high filling of channels. However, their results indicate that the three phase contact line is defined from the top of the channel wall and is kept constant while changing the liquid flow rate. They compared experimental results with predictions of well known model of Nusselt and Kapitza for large scale falling films. They concluded that these correlations were not applicable as they do not take into account capillary forces. Until now, only rounded channels have been considered. But some works have been done using rectangular channels (Zhang et al., 2010; Anastasiou 2013). In this latter case, the liquid film profile is different (Figure 1.14). The three phase contact line can be located on the wall of the channel.

Figure 1.14 : Liquid film profile in rectangular channel (green line). The dashed red line represents the gas-liquid interface

in the case of rounded channels. The dashed black line is the bottom of a channel wall with same dimension as the rectangular one

Thus, tools developed for rectangular channel may be only valid for this geometry but this point remains to be addressed.

For gas-liquid mass transfers, growing literature is available providing experimental results and models (Zhang et al., 2009; Mhiri, 2009; Sobieszuk et al., 2010; Al-Rawashdeh et al., 2010; Ho et al., 2012). Impacts of liquid film thickness, liquid velocity, channel size or gas chamber size are discussed. But those studies are limited to homogeneous reactions (i.e gas absorption in liquid). In cases other than CFD studies, the channel geometry was assimilated to a square one to estimate the liquid film thickness.

Even if this approach is descriptive enough for gas-liquid systems, nothing is mentioned about gas-liquid-solid system but one might expect some differences. What is the impact of parameters described above on mass transfers? What is the impact of catalyst coating on liquid distribution? Which model can be used to design such a reactor? All those questions remain unanswered.

Milli-foam reactor

Solid open cell foams (OCF) are highly porous materials which consist in interconnected ligaments so called struts and defining a pore network. Even though the easiest way to represent pore size is the pore per inch number (PPI) or pore density, other geometrical parameters are required for a better description. Indeed, foams can present different geometrical features according to the solid content, process of fabrication and the nature of foam. Commonly, the mean cell, window, and struts diameters are used (Figure 1.15).

Cell diameter

Window diameter

Strut diameter

Cross section view of the strut

solid Internal porosity Cell diameter Window diameter Strut diameter Cross section view of the strut

solid

Internal porosity

Figure 1.15 : Illustration of main geometrical properties (adapted from utlramet4). The cross section of the strut reveals an internal closed porosity

PPI numbers typically range from 5 to 100 ppi with porosities between 0.75 up to 0.97. Various materials are possible: metal alloys, ceramics or even polymers. Depending on the porosity, the shape of struts can be different: at low solid content, struts are triangle shaped (Figure 1.15), getting more circular upon increasing solid content

Foams have been investigated for many applications such as insulating materials or lightweight structures amongst others (Gibson and Asbhy, 1988). In the particular field of chemical engineering, they found application as heat exchanger, solar receiver, gas filter, packing column or catalyst support. Review on this field is proposed by Twigg et al. (2007).

Extensive researches have been done on gas-solid system using foam as catalyst carrier (Edouard et al., 2008; Giani et al. 2005, Mahjoob and Vafai, 2008). In gas-liquid-solid systems, Stemmet (2008) was probably the first to study the use of foams. He investigated many operating modes (downflow, upflow, co-current & counter current), pressure drop, hydrodynamics and mass transfers. The results indicated a high potential for reacting purposes as gas-liquid mass transfer coefficients (0.1 -1s-1) are one order of magnitude higher than conventional packed bed for low pressure drop. Subsequently, many other works followed on similar reactor configurations (Wenmakers, 2009; Edouard et al. 2008b; Saber et al., 2012; Mohammed et al., 2013). The use of foams as stirrers has also been proposed by Tschentcher et al. (2010).

The high porosity of OCF combined with high specific surface area gives a clear advantage over other packing. Wenmakers (2009) illustrates differences with other packings (Figure 1.16). For foam with PPI number above 40 PPI, specific surface area values outperform those found in monoliths while random or structured packing are far below. Only spherical particles can reach such high values but for much lower porosities and smaller particles which mean high pressure drop. A further advantage lies in that foam structure generates more turbulences compared with monoliths as struts and windows structures stand in the flow way.

4

Figure 1.16 : Specific surface area against porosity for various packing (adapted from Wenmakers, 2010)

Nevertheless, such structures offer rather low effective surface available for catalyst deposition because of the absence of micro porosity in most cases (metal foams). Washcoating with a porous solid can however solve this problem. That was demonstrated by Wenmakers (2009) using carbon nanofibers.

Use of structured support in small channels:

Based on above discussions, it has been shown that on one side foams offer high potentialities for gas-liquid-solid reactions. Even though foam structures present milli or sub-milli scale dimensions, flow patterns are close to those found for conventional packed bed. Even if foam structures will provide higher gas-liquid mass transfers, liquid-solid mass transfers will not benefit from the tortuous path encounter in sphere packing.

On the other side, it has been previously shown that in small channel, Taylor flow presents very good hydrodynamic properties such as narrow RTD, high G/L interfaces… and so on. In the study on μ-packed bed reactor, Losey (2001) reported on evidences about a segmented flow in the micro packed bed. Similarly, de Loos et al. (2010) stated that Taylor flow is conserved in a channel filled with organised pillars under certain velocity conditions. Thus, the purpose of this thesis is to evaluate the combination of OFC and structured flow in small channels.

1.4. Scope and outline of the thesis

This thesis describes the research undertook on the use of structures at milli/micro scales in reactors to enhance global performances. The research presented in this work focuses on two promising concepts: a falling film micro reactor for gas-liquid-solid reactions and the use of foams as catalyst support in a milli channel submitted to a structured gas-liquid flow. Both parts focus on gaining reactor knowledge such as hydrodynamics and mass transfers.

The first part of this thesis is dedicated to answering some of these questions about FFMR reactor at pilot scale. In chapter 2, liquid film profiles and thicknesses are studied with microscopy to propose suitable correlations. These results are coupled with mass transfers experiments to end with a mass transfer model. In Chapter 3 are discussed the impact of channel dimensions and catalyst coating. A more detailed correlation for gas-liquid-solid mass transfers including channel size is proposed and discussed.

The second part investigates the use of metal foams as catalyst support in milli channel submitted to Taylor flow. In chapter 4 are discussed the existence of Taylor flow in the foam packed bed and insights in hydrodynamic features are revealed. Correlations are proposed for the estimation of reactor properties.

In chapter 5, estimation of mass transfer performances for OCF is given in terms of energy input and mass transfers. The influence of foam morphology on mass transfers and flow patterns is assessed. Finally, a comparison with other technologies is proposed to assess the good potential of such approach of a double scale flow structuration.

1.5. References:

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Anastasiou A.D., Makatsoris C., Gavriilidis A., Mouza A. A., 2013, Application of l-PIV for investigating liquid film characteristics in an open inclined microchannel, Experimental Thermal and Fluid Science, 44, 90–99

Chambers R.D., Fox M.A., Holling D., Spink R.C.H, Sandford G., 2001, versatile thin film gas-liquid multichannel microreactors for selective direct fluorination, Lab on Chip, 1, 132-137

Cybulski A., Edvinsson R.K., Irandoust S., Andersson B., 1993, Liquid-phase methanol sysntehsis: modelling of a monolithic reactor, Chemical Engineering Science, 48, 3463-3478.

Cybulski A., Moulijn J.A., 2006, Structured Catalyst and reactors, 2nd ed., p.359, CRC Press Taylor & Francis

De Loos S. R. A., van der Schaaf J., Tiggekaar R.M., Nijhuis T.A., de Croon M. H. J. M., Schouten J. C., 2010, Gas liquid dynamics at low Reynolds numbers in pillared reactangular microchannel, Microfluidics and Nanofluidics, 9, 131-144 Donazzi A., Maestri M., Michael B.C., Beretta A., Forzatti P., Groppi G., Tronconi E., Schmidt L.D. Vlachos D.G., 2010,

Microkinetic modeling of spatially resolved autothermal CH4 catalytic partial oxidation experiments over Rh-coated foams, Journal of catalysis, 275, 2, 270-279

Dudukovic P.M., Larachi F., Mills P.L., 2002, Multiphase catalytic reactors: a perspective on current knowledge and future trends, Catalysis Reviews, 44, 1, 123-246

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Chemical Engineering Journal, 144, 299-311.

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Developement of a monolith-based process for H2O2 production: from idea to large-scale implementation, Catalysis

Today, 69-, 247-252.

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Hipolito – Fernandes A.I., 2010, Étude des phénomènes de transport dans un réacteur catalytique pilote type filaire, phD thesis, Claude Bernard Univeristy, Lyon

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Jähnisch K., Baerns M., Hessel V., Ehrfeld W., Haverkamp V., Löwe H., Wille C., Guber A., 2000, direct fluorination of toluene using elemental fluorine in gas /liquid microreactors, Journal of fluorine chemistry,105, 1, 117-128

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Chemical Engineering Science, 56, 6015 - 6023

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chimiques à fortes contraintes, PhD thesis, Université Claude Bernard Lyon 1.

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Losey M. W., Schmidt M. A., Jensen K. F., 2001, Microfabricated multiphase packed-bed reactors: characterization of mass transfer and reactions, Industrial and Engineering Chemistry Research, 40, 2555-2562

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Journal of Heat and Mass Transfers, 51 3701-3711.

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Packings, Chemical Engineering Journal, doi: http://dx.doi.org/10.1016/j.cej.2013.07.024

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dynamic–static liquid mass transfer in trickle flow reactor containing ȕ-SiC open-cell foams, Chemical Engineering

Journal, 185–186, 294-299

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and Processing, 48, 820-824

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Stemmet C.P., 2008, Gas-Liquid-Solid foam reactors: Hydrodynamics and mass transfer, PhD thesis, Technical University of Eindhoven

Tschentscher R., Nijhuis T. A., van der Schaaf J., Kuster B. F. M., Schouten J. C., 2010, Gas–liquid mass transfer in rotating solid foam reactors, Chemical Engineering Science 65, 472-479

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modeling, PhD thesis, Technische Universiteit Eindhoven

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Chemical Engineering Science, 56, 1265-1273

Yeong K.K.,Gavriilidis A., Zapf R., Hessel V.,2003 ,Catalyst preparation and deactivation issues for nitrobenzene hydrogenation in a microstructured falling film reactor, Catalysis Today , 81,4, 641-651

Yeong K.K.,Gavriilidis A., Zapf R., Hessel V., 2004, Experimental studies of nitrobenzene hydrogenation in a microstructured faling film reactor, Chemical Engineering Science, 59, 3491-3494

Zhang H., Chen Q., Yue J., Yuan Q., 2009, Hydrodynamics and mass transfer of gas–liquid flow in a falling film microreactor, AIChE Journal, 55, 5, 1110-1120

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Mass transfer characterization of a microstructured falling film at pilot

scale

This chapter has been adapted from the following publication:

Tourvieille J-N, Bornette F., Philippe R., Vandenberghe Q., de Bellefon C., 2013, Mass transfer characterization of a microstructured falling film at pilot scale, Chemical Engineering Journal, 227, 182-190

Modifications in mass transfer results have been brought to the original publication because of miscalculations. Thus, the film model was no longer suitable to represent the new experimental data. These modifications will be the subject of an upcoming erratum and are included in this chapter.

Abstract

This work presents a first approach in the study of gas-liquid-solid mass transfer in a microstructured falling film at pilot scale based on experimental results of film morphology. The liquid film thickness is first investigated by fluorescence confocal microscopy by varying the physical properties of solvents such as viscosity and surface tension. A correlation is proposed that reveals a low effect of the capillary number Ca. Results are coupled with experimental gas-liquid-solid mass transfer determination using the catalytic hydrogenation of Į-methylstyrene over 5% Pd/Al2O3. The overall gas-liquid-solid mass transfer coefficient Kov appears to be severely impacted by the liquid film thickness with values in the range from 5.0 to 2.9 s-1 at high and low flow rates respectively.

Key words: Falling film, Micro-structured reactor, Film thickness, Confocal microscopy, Mass transfer

2.1. Introduction

The interest of microstructured reactors for process intensification has been demonstrated through several applications (Roberge et al., 2005; Kashid and Kiwi-Minsker, 2009) over many years. They are generally characterized by high surface to volume ratios leading to enhanced mass and heat transfer performances compared with traditional reactor technologies.

One of these reactors is the well-known micro-structured falling film (FFMR-Standard) produced by IMM (Figure 2.1.a) and described elsewhere (Jähnisch et al., 2000; Hessel et al., 2005).

In this continuous reactor, a liquid is falling down a vertical grooved plate. The combination of capillary forces and small dimensions stabilizes the gas-liquid interface and thin liquid films below 100μm can be obtained. Two operating regimes can be achieved. A gas can circulate over the liquid phase in counter or co-current flow and both flows are controlled separately. Gas-liquid-solid reactions are also considered by coating a channel with a catalyst (Gavriilidis et al., 2003). Combined with a heat exchanger on the back of the plate, this provides a really

interesting tool for very demanding reactions. A wide range of applications have been studied in FFMR such as fluorination (Jähnisch et al., 2000), hydrogenation (Yeong et al., 2004) separation of binary system (Kane et al., 2011) or CO2 absorption. For this latter, numerical models have been proposed (Zanfir et al., 2005; Al-rawashdeh et al., 2008)

Despite interest sparked off by this improved contactor, only lab scale throughputs (in most cases about 1ml.min -1

) are achieved. For industrial purposes, investigation of scale up is needed. From this perspective IMM has developed a tenfold scale up falling film micro reactor (FFMR-L) presented in (Figure 2.1.a, b). Coating the micro channels with a catalytic phase can also be of great interest for pharmaceutical applications.

Figure 2.1 : (a) FFMR-Standard and FFMR-L (IMM) (b) Reaction plates (c) Slice view of coated channel (Stavarek et al.,

2008)

Assessment, control, and prediction of mass transfer performances are a fundamental step for successful industrial implementation. Considering falling film technology, the key parameters for mass transfer control are the liquid film thickness and hydrodynamics of the liquid phase.

Some works have been devoted to this latter point. The use of smart interfaces such as herringbones structures has shown that an interesting improvement can be obtained in mixing quality leading to better conversion for G/L systems (Ziegenbalg et al., 2010; Rebrov et al., 2012; Al-rawashdeh et al., 2012).

At the scale of micro-structures, viscous and capillary forces are predominant with a Ca number below 10-3. Understanding the impact of these forces on mass transfer performance is of great interest and correlations, empirical or otherwise, are useful to gain more reliable predictions. Most of the works dealing with mass transfer assessment have carried out at the lab scale. In their study, Yeong and co-workers proposed a method for study of liquid film thickness by confocal microscopy (Yeong et al., 2006) for different solvents on different supports but did not completely clarify the role of capillary forces. They also discussed the validity of correlations established for classical falling film technology. They defined an average film thickness taking into account microchannel morphology and compared with that calculated by the Nusselt and Kapitza correlations (Nusselt, 1916; Kapitza, 1964) they found quite good agreement despite some discrepancies at low flow rates attributed to a curvature degree to steep to be measured. Zhang et al mentioned the impact of viscosity, contact angle and surface tension on G/L absorption of CO2 (Zhang et al., 2009) in a home made FFMR. This point was considered by Ho et al. (2011) through CFD computation of the two phase flow where they discussed impact of surface

(a)

tension and viscosity on liquid flow rates. However, the impact of these predominant forces on mass transfer was not examined for gas-liquid-solid systems.

Only one experimental study has tried to link liquid film thickness with mass transfer for a gas-liquid-solid reaction (Yeong et al., 2004). Nitrobenzene hydrogenation was performed to experimentally determine an overall gas-liquid-solid mass transfer coefficient but the results were not conclusive. It appeared that the reaction regime was not completely under mass transfer control.

Therefore, the present work proposes a first approach to determine overall external mass transfer performances in a pilot scale version of a microstructured falling film (Vankayala et al., 2007). Toward these objectives, the impact of viscosity and capillary forces on liquid film thickness is first investigated and results are coupled with the Į-methylstyrene hydrogenation on Pd/Al2O3. Liquid distribution is also considered.

Previous authors demonstrated the interest of confocal microscopy in reflection mode for determining liquid film thickness across a microchannel section in the standard version of FFMR (Yeong et al., 2006). Several slices at different focal planes spaced by a calibrated length are compiled to reconstruct the whole object (Figure 2.2). However, measurements were only possible when the gas-liquid interface were flat enough to avoid reflection issues. This means that at low flow rates, only focal points around the centre of microchannel can be observed. As mentioned by Seeman et al. (2005) the location of the three phase contact line is a key parameter for description of the morphology of the liquid film into microchannels. This information coupled with the liquid film thickness at the centre (įc) of the micro channel is the main information required to describe film morphology.

Then, depending on image quality, a critical lack of information may arise on the film thickness near edges as well as on the location of the three phase contact line. In our work we propose to complete this method by working in fluorescence mode as emission is isotropic and its intensity easily tunable by adjusting fluorescent dye concentration. Gas flow Liquid flow Micro channels Gas flow Liquid flow Gas flow Liquid flow Micro channels 4μm 800 μm y z x 4μm 800 μm y z x

2.2. Methods, experimental set-up and procedures

2.2.1. Visualisation of the liquid film

2.2.1.1. Reactor aspects

For practical reasons (weight, tautness aperture of viewing windows) the confocal experiments were performed with a PEEK mock-up, keeping all dimensions of the original reactor equal (Figure 2.3.a). This material has been chosen for its excellent mechanical strength and its good chemical inertia. A gear pump (Tuthill) coupled with a Coriolis liquid flow meter (Bronckhorst mini-coriflow) has been used to feed liquids without any pulsation. Liquid distribution is ensured by an evenly perforated metal tube and inserted in the inlet liquid chamber. At the outlet of the mock-up, a peristaltic pump has been placed to avoid liquid accumulation. The reactor can be operated either open to the liquid or batch wise by means of a recirculation loop (Figure 2.3.b). The whole system was settled on a motorized translation stage Standa 7 MT100. The confocal microscope (Leica SP5) has been fitted with a 90° bending mirror equipped with a 10X lens. In these conditions two contiguous micro channels are observed. The fluorescent dye used was Rhodamine 6 B diluted at 10-4 mol/L. Several solvents were used exhibiting different viscosities and surface tensions: ethanol (absolute VWR Prolabo), isopropanol (VWR prolabo, 99.9%), and demineralised water. Only one grooved plate without catalyst and with the micro channel specifications given in Table 2.1 was used.

a)

a)

Figure 2.3 : Microscopy set –up. (a) 90° bending mirror and reactor laying on translation stages. (b) : (1) Liquid tank, (2)

gear pump with Coriolis flow meter, (3) Syringe, (4) Peek mock-up, (5) Peristaltic pump 2.2.1.2. Procedure

Efficient wetting of the plate and equal liquid distribution over the whole reactor was achieved by feeding pure isopropanol at a high flow rate to overflow the channels. Priming the reactor with this procedure is necessary to ensure reproducible film morphology. Then, isopropanol is replaced by the considered solvent containing fluorescent dye while the flow rate is decreased to the set point and the system is switched to closed circuit. Preliminary tests showed a severe decrease of the film thickness for ethanol over the height of the plate. This means solvent evaporation has a significant impact. Measurements are therefore carried out at the closest